The present invention relates to the problem of providing antireflective surface layers or coatings on glass, and is specifically concerned with methods for providing stable antireflective surface layers on chemically-durable borosilicate glasses.
An inexpensive method of providing an antireflective film or layer on glass could be used in the production of non-glare glass for picture frames, television screens, eyeglasses and lenses, windshields, instrument panels, display windows and the like. One area of recent interest, requiring a method of greatly reduced cost and complexity, is the application of antireflective coatings to glass envelopes for solar energy collectors. It has been recognized that reflection losses from the envelope can substantially affect collector efficiency.
The reflection of light from a surface depends not only on the angle of incidence of the light, but also upon the refractive index of the material forming the surface. For glass, with an average refractive index of about 1.5, the reflectance from a single surface is about 4% for light impinging at incident angles of less than about 50.degree.from the normal.
There presently exists a highly developed technology for producing antireflective coatings on optical surfaces. In general, the highest quality coatings are produced by vacuum deposition techniques. However, while such techniques are suitable for the batch treatment of small articles such as lenses, they are expensive and not readily adaptable to the continuous production of antireflective coatings on large articles. Particular difficulty is encountered in attempting to provide an antireflective coating on the surfaces of articles of complex shape, such as the interior walls of a glass envelope for a solar energy collection device.
It is not difficult to produce an antireflective film having zero reflectance in a narrow range of wavelengths. However, broad-band antireflective coatings generally consist of multiple layers, often as many as fifteen or more, in order to achieve low reflectance in both the visible and infrared spectral regions. Coatings having a graded refractive index appear to be effective in producing the desired broad-band antireflective properties.
It has long been known that the reflectance of glass surfaces could be reduced by etching techniques. Such etching can remove leachable components from the glass, leaving a skeletonized porous surface having a lower refractive index than the bulk glass. If etching is permitted to proceed only to an extent sufficient to provide a skeletonized surface layer of a depth approximating an odd multiple of one-fourth the wavelength of the light to be transmitted, then reduced reflectance of this light by the treated surface is observed.
Most of the known etching processes involve complex etching solutions and procedures which must be designed specifically for the type of glass composition to be treated. U.S. Pat. No. 2,348,704 to Adams, for example, describes a procedure for treating barium crown glass by removing the alkali, alkaline earth, and other bivalent metal oxides from the glass, and thereafter treating the glass with hydrofluoric acid to enlarge the pore structure of the residual siliceous layer.
U.S. Pats. Nos. 2,486,431 to Nicoll et al. and 2,490,662 to Thomsen describe methods for treating soda-lime glasses (e.g., window glasses) or optical crown glasses with complex, silica-saturated solutions of fluosilicic acid, in order to provide antireflective surface films thereon. However, these methods are not effective to produce efficient antireflective surface layers on borosilicate glasses.
As noted by L. Holland in The Properties of Glass Surfaces, Wiley & Sons, New York, (1964) on pages 155 and 165, acid etching processes do not typically produce good antireflective films on the surfaces of chemically-durable borosilicate glasses. Holland points out that the production of antireflecting films by chemical etching is presently deemed of little practical value in view of the weak and optically inefficient nature of the films so produced.